1. Field of the Invention
This invention relates to polyvinyl alcohol (PVA) membrane materials and a method of producing PVA membranes having a uniform, microporous structure and an improved permeability.
2. Description of the Prior Art
In recent years, porous membranes produced by various methods and intended for use in the separation of minute particles having sizes not greater than one micron by filtration have been reported. Such membranes have been produced by many different techniques, for example: membranes have been produced by dissolving a polymer in a mixture of a good solvent and a poor solvent for the polymer and casting the resulting mixture into a thin film; membranes have been produced by mixing a substance soluble in a polymer solution and thereafter extracting the substance from the resulting shaped article; membranes have been produced by sintering together a layer of polymer particles; and membranes have also been produced by stretching a film to form a fibrillated structure or microporous structure. These membranes, however, have both advantages and disadvantages which are attributable to the methods of formation thereof.
While PVA membranes for separation purposes have now become of interest due to special properties of the polymer especially the hydrophilic property thereof, porous PVA membranes have been rarely reported.
It is known that porous PVA sponge can be produced by mixing starch or dextrin together with formalin and an aqueous PVA solution, thereby causing formalization and at the same time swelling the starch or dextrin particles, and, after the formalization, removing the starch or dextrin by washing the product with water. The sponge prepared by this method, due to the method itself, exhibits pore sizes as large as several microns or more, generally more than 10 microns, and therefore is not suited for separation of more minute particles by filtration. Another drawback of the sponges prepared by this method is that they generally exhibit non-uniform pore size distributions. It is not possible to produce hollow fiber membranes by spinning the polymer solution because the formalization reaction mentioned above is effected over a fairly long period of time.
U.S. Pat. No. 4,073,733 discloses uniform, porous PVA membranes having specific micropores made by a process comprising dissolving a polyalkylene glycol (PAG) into a PVA aqueous solution, coagulating the resultant solution in a coagulation bath to form a membrane and, removing PAG by extraction during or after coagulation. The membranes obtained by this method have average pore sizes of 0.02-2 microns and show excellent performance in the separation of minute particles. It is not possible, however, to produce membranes having larger pore diameters by this method.
Thus, porous PVA membranes having relatively large pore sizes and relatively small pore sizes, respectively, are known and commercially available, but no method for producing membranes wherein the pore sizes can be varied, as desired, from large to very small has so far been provided. Today the demand for membranes has significantly increased and membrane performances are required to be suitable for widely different applications. Under these circumstances, there is a great need for membranes having a wide variety of pore sizes.
In addition to pore size, narrowness of pore size distribution is another important factor to the performance of porous membranes. The narrower the pore size distribution, the better the cutoff performance of the membranes. Good cutoff is desirable for membranes used in filtration applications.
As mentioned above, known porous PVA membranes having relatively large pore sizes do not show narrow pore size distribution and cannot be used in fine filtration applications; whereas known porous PVA membranes having relatively small pore sizes can still be improved in the narrowness of the particle size distribution, although they exhibit fairly good pore size distribution.
The present inventors have now found that excellent PVA membranes without any of above problems can be obtained by the method of the present invention.